BIOCHEMISTRY AND MOLECULAR-BIOLOGY OF PLANT SULFOTRANSFERASES

It is now well established that, in mammals, sulfate conjugation const itutes an important reaction in the transformation of xenobiotics and in the modulation of the biological activity of steroid hormones and n eurotransmitter (1, 2). The presence of a sulfate group on some molecu les can also be a prerequisite for their biological function. For exam ple, it is well known that the sulfate groups are directly involved in the molecular interaction between heparin and antithrombin III (3). I n plants, sulfation also seems to play an important role in the interm olecular recognition and signaling processes, as indicated by the requ irement of a sulfate moiety for the biological activity of gallic acid glucoside sulfate in the seismonastic and gravitropic movements of pl ants (4), and of Nod RM1 in the cortical cell division during early no dule initiation in Rhizobium meliloti-alfalfa interaction (5). In addi tion, recent studies indicate that flavonoid conjugates, including the sulfate esters, may play a role in the regulation of plant growth by strongly binding the naphthylphthalamic acid receptor, thus blocking t he quercetin-stimulated accumulation of the auxin phytohormone (6). Al though several sulfated metabolites are known to accumulate in a varie ty of plant species (7), the study of enzymes that catalyze the sulfat ion reaction in plants lagged considerably compared to those conducted with their mammalian homologs. This apparent lack of interest may hav e been because the function of plant-sulfated metabolites is difficult to predict, since their accumulation is often restricted to a limited number of species. Despite this limitation, several plant sulfotransf erases (STs) have been characterized at the biochemical level, and the cDNA clones encoding six plant STs have been isolated. Based on seque nce homology, the plant ST coding sequences are grouped under the SULT 3 family, also known as the flavonol ST family. This review summarizes our current knowledge of the plant STs and focuses on the functional significance of the sulfate conjugation in plant growth, development, and adaptation to stress.